>>3812020Selective Gaussian Blur with radius 20 or something and max delta 0.20, few tries with different Curves and Posterize 4 levels until result looked bit like yours. I think I used Curves roughly like this. Still Gimp.
Larger Blur Radius might give less convoluted edges and curves adjusted to change shape and location of posterzed color areas but it's your photo, you fiddle with it.
